1. Field of the Invention
The present invention relates to a power supply unit for an electric motor vehicle capable of obtaining an accessary power supply from a running power supply kept in the floating state by way of an input-output insulation type DC-DC converter, wherein one end of the accessary power supply is connected to the body grounding. In particular, the present invention concerns a power supply unit for an electric motor vehicle in which a power or signal for starting an accessary power supply is obtained from a running power supply by way of a main switch such as an ignition key, wherein a switch having a low withstand voltage is usable and a leak current is prevented from flowing the running power supply even when the insulation between the contact of the main switch and the body grounding is lowered.
2. Description of Background Art
In general, an electric motor vehicle includes a running power supply and an accessary power supply. The running power supply is adapted to mainly supply power to a running motor. The running power supply includes, generally, a high voltage from several tens to several hundreds of volts (V), and is thereby kept in the floating state, one end of the power supply is not connected to the body grounding.
The accessary power supply is adapted to supply power required for various controllers and lighting devices, and has generally a low voltage of 12 to 24 V or less. Accordingly, one end of the accessary power supply is connected to the body grounding for lowering the grounding impedance of the accessary power supply; and a body made of metal is employed as the return path of the drive current for a lighting device thereby omitting the wiring used for the current return path.
An electric motor vehicle is known which includes a special battery as an accessary power supply separately from the running power supply. However, to reduce the space for mounting the battery and the weight of the battery, there has been mostly used an electric motor vehicle of a type having an input-output insulation type DC-DC converter for obtaining an accessary power supply by the step-down conversion from a running power supply.
FIG. 32 is a circuit diagram showing the construction of a prior art power supply unit for an electric motor vehicle wherein an accessary power supply is obtained by the step-down conversion from a running power supply kept in the floating state.
In a prior art power supply unit 1 for an electric motor vehicle, a main switch 2, for example an ignition key switch, is used as the starting switch, and a power supply voltage VM of a running power supply 3 is supplied to a starting control terminal 4a of a DC-DC converter 4 of an input-output insulation type by way of the main switch 2, thereby obtaining a step-down accessary power supply VB on the output side of the DC-DC converter 4. A starting circuit 5 is formed of, for example, a relay 6. An exciting winding 6a of the relay 6 is energized, and the running power supply 3 is supplied to the input side of a DC-DC conversion circuit 4A by way of a contact 6b of the relay 6, to start the DC-DC conversion. The accessary power supply VB is connected, for example, on the negative electrode side, to the grounding body.
When the accessary power supply VB is raised, the lighting device system of a vehicle is in the operable state, and various controllers are started to be operated. For example, on the basis of an energizing command from a motor drive controller, each semiconductor switching element (not shown) within a power inverter is driven, to supply power to a running motor, thus making the vehicle operable.
The switching of the DC-DC converter 4 may be directly performed by the main switch 2, without the relay 6. However, in this case, a switch with a large current capacity is required; and further, a contact tends to be damaged by switching a relatively large current.
For this reason, as shown in FIG. 32, to switch the power supply on the input side of the DC-DC converter 4, a relay 6 is driven by way of the main switch 2; or semiconductor switching elements (not shown) are driven in place of the relay 6.
However, since the running power supply 3 has a high voltage, the main switch 2 requires a high withstand voltage.
In another power supply unit 11 for an electric motor vehicle shown in FIG. 33, a starting power or a starting signal is supplied from a battery unit 3a constituting part of the running power supply 3 including a plurality of battery units 3a to 3n connected in series to each other to a starting control terminal 4a of the DC-DC converter 4 by way of the main switch 2, thus starting the DC-DC converter 4. With this construction, it is possible to use the main switch having a low withstand voltage between contacts.
On the other hand, the main switch 2 for starting the accessary power supply is an important key for controlling the turn on/off of the whole electrical system of the electric motor vehicle. Accordingly, as shown in FIG. 34, the opening/closing of contacts is performed by inserting a key in a key insertion hole 2K and turning the key. Further, a lock pin 2L for restricting the operation of the steering system is interconnected therewith. A body 2a of the main switch 2 composed of the key switch is made of metal, and in general, the main switch 2 is mounted on the metal portion of a vehicle forming the body grounding.
As described above, in the prior art power supply unit, the main switch is turned on to supply a starting power or starting signal from the running power supply, thus raising the accessary power supply. However, as shown in FIG. 34, in view of the fact that the structure of the main switch 2 and the contact are in close proximity to the body 2a of the switch, the body 2a of the main switch 2 forms the body grounding. Accordingly, the insulating resistance to the body grounding tends to be reduced due to water permeation, equivalent insulating resistance value R1 of the main switch 1 shown in FIG. 33 is reduced. Moreover, when the insulating resistance between the positive electrode side of the running power supply and the body grounding is simultaneously reduced, equivalent insulating resistance R2 in FIG. 33 between the running power supply 3 and the body grounding is reduced, as shown by the broken line of FIG. 33, a leak current LC flows, through the main switch 2, along the route of the high voltage side of the running power supply 3, equivalent insulating resistance R2, body grounding equivalent insulating resistance R1 of the main switch 2-low voltage side of the running power supply 3. This causes disadvantages in generating unnecessary current consumption from the running power supply 3 and applying a high voltage to the contact of the main switch 2.
To solve the above problems, the present invention has an object to provide a power supply unit for an electric motor vehicle capable of using a main switch having a low withstand voltage, and blocking a leak current flowing along the starting route of an accessary power supply containing the main switch.
To achieve the above object, according to the present invention, there is provided a power supply unit for an electric motor vehicle including a running power supply for supplying power to a running motor which includes a plurality of batteries connected in series to each other, and which is kept in the floating state with respect to the body grounding. A step-down type DC-DC converter is provided in which the input side is insulated from the outside. A diode and a main switch are at least provided in this order from a battery constituting part of the running power supply, to supply a starting power or a starting signal to the DC-DC converter for controlling the DC-DC converter to be operated, thereby supplying power from the running power supply to the input side of the DC-DC converter and obtaining a stepdown accessary power supply on the output side of the DC-DC converter.
When a main switch is turned on, a starting power or starting signal is supplied to a DC-DC converter by way of a diode or the main switch, to start the operation of the DC-DC converter, thus supplying an accessary power supply.
The diode is interposed along the direction of supplying a starting power or starting signal from the battery of the running power supply to the DC-DC converter, so that it can prevent the current flowing in the reverse direction.
For example, even if the insulation of the main switch is reduced due to the water permeation and the insulation between the running power supply and the body grounding is reduced, a leak current flowing in the direction from the body grounding to the running power supply is blocked by the diode. Therefore, it becomes possible to prevent the generation of a leak current, to eliminate unnecessary current consumption from the running power supply, and to prevent the high voltage applied to the contact of the main switch.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.